The present invention relates to the field of seismic sources for evaluating geophysical structures. More particularly, the invention relates to an improved seismic source using a pressurized fluid within a slotted housing to generate seismic source energy.
Seismic sources are operated in open water and downhole in boreholes to generate acoustic source energy. In marine geophysical operations, seismic vessels tow vibrators, air guns, explosives, and other acoustic projector techniques to generate seismic source energy. The seismic source energy is represented by a pressure pulse in the water which travels downwardly through the water and underlying geologic structures and is reflected from interfaces between the geologic structures. The reflected signal impulses return to the water column and are detected with sensors towed behind the seismic vessel or laid on the water bottom. To evaluate the geologic structures proximate to a borehole, acoustic source energy is generated a the surface or downhole and is detected with sensors located at the surface, in adjacent boreholes, or in the same borehole.
Marine seismic operations use high powered acoustic signals near 190 dB/Hz re micro Pascal in a low frequency band between 5-120 Hz. High power signals penetrate deep within subsurface geologic structures, and low frequency signals experience less attenuation than higher frequency signals. High power, low frequency source signals for marine geophysical operations are typically generated with air guns or other acoustic sources. U.S. Pat. No. 3,896,889 to Bouyoucos (1975) disclosed a mass oscillation system for generating acoustic source energy in water. Other devices generate an acoustic signal by transmitting high velocity water jets into the underwater water environment. U.S. Pat. No.4,131,178 to Bouyoucos (1978) and U.S. Pat. No. 4,153,135 to Bouyoucos (1979) disclosed a moveable piston for generating high velocity water jets. U.S. Pat. No. 4,234,052 to Chelminski (1980) disclosed another liquid jet acoustic source system. Improvements to liquid jet acoustic source systems were disclosed in U.S. Pat. Nos. 4,695,987 to Buoyoucos (1987) and 4,753,316 to Buoyoucos et al. (1988).
Another type of seismic source system releases compressed air through various mechanical firing systems into the water. U.S. Pat. No. 4,180,139 to Walker (1979) disclosed one type of air gun, and U.S. Pat. No. 4,285,415 to Paitson (1981) disclosed a mechanism for controlling the discharge of compressed air. U.S. Pat. No. 5,228,010 to Harrison (1993) disclosed a shuttle air gun for generating acoustic source energy.
Slotted cylinders have been used in different applications to generate relatively high frequency, low power pressure pulses in water. Such cylinders use complicated piezoelectric crystal stacks attached to the interior wall of a slotted cylinder and require numerous electrical power connections. Acoustic vibrators have been used in submarine operations to detect and to locate the position of vessels and underwater objects, however such vibrators do not operate at the power levels and frequency necessary in geophysical seismic operations. An example of a high frequency, low power system is described in U.S. Pat. No. 3,875,552 to Hogman et al. (1975) wherein sonar signals were transmitted from a mobile, underwater target. Another type of marine acoustic output generator was disclosed in U.S. Pat. No. 5,875,154 to Dechico (1999), wherein stacks of piezoelectric elements were arranged between first and second end caps of a radiator.
In addition to the high frequency, low power electromechanical transducers described above for submarine operations, slotted transducers have been used in loud speakers and in underwater sonar applications. Proposed uses for such transducers include pile drivers, trench diggers, gravel packers, replaceable knives or drills or surgical blades, sonic tools in oil wells, and sonobuoy and sonar installations. U.S. Pat. No. 4,257,482 to Kompanek (1981) disclosed a sonic gravel packing method for downhole boreholes.
Other forms of electromechanical transducers have been developed. U.S. Pat. No. 4,220,887 to Kompanek (1980) described a slotted electromechanical transducer having a resilient member in the slot for prestressing the transducer and for preventing contact between adjacent transducer ends. U.S. Pat. No. 4,651,044 to Kompanek (1987) specifically attempted to produce large amounts of power at low frequency, however the frequency range was in the order of "several kilocycles". U.S. Pat. No. 5,122,992 to Kompanek (1992) disclosed a transducer member having a closure member extending in a U-shaped configuration. The length of the closure member defined the bandwidth of the vibration frequency produced. In U.S. Pat. No. 5,267,223 to Flanagan et al. (1993), a compliant cover was bonded to a transducer shell.
Conventional seismic source technology does not efficiently provide high power, low frequency source energy from a simple operating system. Piezoelectric crystal systems require electronic systems having electrical sources susceptible to failure, and such failure can significantly complicate downhole wellbore operations. Large downhole pressures can generate failure of piezoelectric components, therefore limiting use of the acoustic source devices in certain applications. There is, accordingly, a need for an improved seismic source generator which can reliably provide acoustic source energy in different operating environments.